Rear wheel suspension systems have been used on a variety of two-wheeled vehicles, including motorcycles, scooters and bicycles, for providing improved rider comfort and increased performance.
Rear wheel suspensions on pedal powered bicycles have become increasingly popular, and generally provide a rider with the benefits of a more comfortable ride and better control over the bicycle. Such bicycle suspension systems improve ride quality by absorbing the shocks incurred from encountering ground obstacles, rather than transmitting them through the frame to the rider. By maintaining greater contact between the tire and the ground, the suspension also provides the rider with better control for accelerating, braking, and cornering.
For a suspension to be suitable for use on a bicycle, it must be efficient. Ideally, a perfect rear wheel suspension would compress only in reaction to ground forces but not to drive-train or braking forces. Unwanted suspension movement resulting from drive train forces wastes rider energy. Bicycle suspensions can be designed to react principally to ground forces, and such that drive-train and braking forces which act thereon are limited, by careful selection of suspension type and geometry.
Several types of rear wheel suspensions exist. One of these suspension systems comprises attaching the rear axle directly to a swing-arm which pivots around a single fixed pivot axis on the forward main frame. In such a system, the pivot point around which the rear wheel axle rotates is simply the pivot point at which the swing-arm is attached to the frame. This type of suspension benefits from being simple and, if the single fixed pivot is correctly placed and the suspension geometry is correctly chosen, this type of suspension can be effective. However, the possible locations for the main pivot are often limited by traditional frame geometry and by the necessity for mounting other components such as the shock absorber and the front derailleur.
Another type of suspension system which is currently growing in popularity, and which enables a rear wheel travel path which defines a quadratic trajectory, comprises a four-bar linkage in which two different linkages or pairs of linkages are attached to the main frame of the bicycle. A third member, to which is attached the rear axle, is engaged with each of these two linkages. In such a four-bar linkage suspension, the center of rotation of the rear axle is not fixed, as in the fixed pivot axis suspensions, and varies with the relative position of the linkages. Thus, as the suspension moves, the instantaneous center of rotation changes. Such a variable pivot point (VPP) system accordingly allows for a non-constant rate of change between the rear wheel axle and the bottom bracket of the main frame portion.
Both of the above types of suspensions have their advantages, however most known suspension designs of either type have associated disadvantages. For example, known single/fixed pivot rear suspensions generally require the shock absorber for the single pivot swing arm to be located relatively high in the main frame of the bicycle and therefore result in a relatively high overall center of gravity of the bicycle. This is disadvantageous in many bicycling applications, particularly when covering steep or mountainous terrain. Further, traditional single pivot suspensions assemblies often have a suspension structure with a center of mass which is off-center relative to the bottom bracket of the frame, resulting in fore-aft weight imbalances. Such weight imbalances can result in reduced maneuverability of the bicycle.
There exists therefore a need for an improved bicycle rear suspension which addresses at least some of the forgoing problems with known designs.